The present invention is directed to sonic distance-measuring devices. It pertains particularly to sonic devices for measuring distances in air, such as the distances between walls in houses.
Numerous tapeless distance-measuring devices are presently available on the market. They greatly facilitate distance measurement, since one person can take a distance measurement with the press of a button while measurements with a conventional tape measure are more time-consuming and are difficult to take without a second person.
Although tapeless distance-measuring devices are relatively easy to use, presently available devices require some care in order to ensure that an accurate measurement is being made. They also tend to be expensive.
These two drawbacks are related. The user of a presently available tapeless measuring device typically aims the device, presses a button, and reads the measured distance on a device display. In many situations, however, it is best for the user to take several measurements, changing the orientation of the measuring device slightly between measurements. The reason for this is that the device may be detecting a target other than that at which the user believes he is aiming the device, and a significant change in the read-out between measurements will indicate to the user that he is measuring different targets. Without such care, the user can easily make a faulty measurement.
One of the reasons for the tendency to detect incorrect targets is that the sound transmitted by the sonic measurement device is of limited directionality. That is, there is a cone in space in which the device will detect objects, and this cone is often wide enough to include not only the desired target but also some false targets. To reduce this problem, the makers of such measuring devices employ large-diameter ultrasonic transducers. The larger the diameter of the transducer is for a given sound wavelength, the narrower the angle of the cone will tend to be. However, the large-diameter transducers are usually expensive because of the effort required to make the entire transducer face vibrate in phase. One way to obtain a narrower cone without using a large transducer is to provide a horn or other sonic "lens" to make a small transducer act as a large one. But such expedients tend to make the device bulky, and they, too, contribute significantly to the expense of the device.